Okadaic acid is known to be a toxic C.sub.38 polyether fatty acid first isolated from marine sponges Halichondria okadaii. Caused by enormous blooms of toxic phytoplankton known as "Red Tides", this toxin is produced and secreted by several types of marine dinoflagellates (plankton) of the genus Dinophysis. Okadaic acid accumulates in marine sponges, mussels, and scallops by filter feeding and has been implicated as the major component responsible for the phenomenon known as diarrhetic shellfish poisoning (DSP). DSP does not appear to be fatal, but its high morbidity rate and worldwide occurrence have made it a serious threat to the shellfish industry and to public health in general.
The first occurrence of diarrhetic shellfish poisoning was reported in the Tohoku District, Honshu Island, Japan, during the summer of 1976 and 1977. In the past decade, okadaic acid has been responsible for tens of thousands of human intoxications throughout Europe, South America, Hawaii, Chile, Indonesia, Japan, the United States and Canada. Between 1976 and 1985, the number of cases in Japan has exceeded 1300 despite the existence of extensive surveillance. In Spain alone as many as 5000 cases have been reported in 1981.
The dinoflagellate Dinophysis acuminata is probably responsible for okadaic acid production in most parts of Europe, while chemical analysis of the toxin isolated from Dinophysis fortii revealed the major component to be 35-methyl-okadaic acid. Accordingly, the compound was named dinophysis-toxin-1 (DTX-1)
In Norway, okadaic acid is found in mussels from the southern regions, while dinophysis-toxin-1 is found in mussels from the southwestern regions of the country. In both cases, Dinophysis acuta and Dinophysis norwegica are suspected of being responsible for the toxin infestation of the mussels.
The lethal potency of okadaic acid is 192 .mu.g/kg when injected intraperitoneally in mice.
Recently, it has also became apparent that okadaic acid has potent tumor-promoting activity. It has been established that okadaic acid is capable of entering cells causing marked increases in the phosphorylation state of many proteins involved in the regulation of carbohydrate and lipid metabolism. Because protein phosphatases PP1 and PP2A are most probably the chief enzymes that reverse the actions of protein kinase C, it is not surprising that a potent phosphatase inhibitor (okadaic acid) should be as potent a tumor promoter as substances (phorbol esters) that activate protein kinase C. Tumor promotion presumably stems from increased phosphorylation of one or more proteins that are substrates for protein kinase C and dephosphorylated by PP1/PP2A.
No diarrhetic shellfish poisoning (DSP) standards to date are available in North America and no established assay exists for their detection. At present a number of countries, including Canada, Norway, Sweden, France and Japan are using suckling mice bioassays. However, this assay carries a considerable margin of error, is extremely tedious to perform and involves use of animals.
A group working on liquid chromatography assays was able to measure fluorescence intensity of the toxins after their reactions with 9-anthryldiazomethane (AGRIC. BIOL. CHEM., Lee S.S. et al., 1987, 51(3), pp.877-881). This method is sensitive but requires sophisticated, expensive equipment and highly skilled and trained personnel. It could never be adapted to field testing.
UBE Industries of Japan have recently introduced a new assay for measurement of okadaic acid (OA) in seafood samples. The assay, ELISA, is based on a competition between solid-phase bound okadaic acid and free okadaic acid for binding to anti-OA murine monoclonal antibody labelled with enzyme peroxidase (European Patent Application published on April 12, 1989 under No. EP-A-311,456).
The assay is very expensive due to the use of okadaic acid as a capture antigen. Okadaic acid is extracted from sponges, collection and processing of which is very expensive. The kit in the present form is not marketable for general, profilatic testing of seafood and plankton by fish industries due to its high cost and complicated sample preparation.
It would be highly desirable to have an assay for the measurement of okadaic acid which would be accurate, sensitive, easy to use and inexpensive.